MCP48FVB21 datasheet(62/84 Pages) MICROCHIP

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MCP48FVBXX

DS20005466A-page 62

 2015 Microchip Technology Inc.

8.2

Application Examples

The MCP48FVBXX devices are rail-to-rail output DACs

designed to operate with a VDD range of 2.7V to 5.5V.

The internal output operational amplifier is robust

enough to drive common, small-signal loads directly,

thus eliminating the cost and size of external buffers for

most applications. The user can use gain of 1 or 2 of

the output operational amplifier by setting the

Configuration register bits. Also, the user can use

internal VDD as the reference or use an external

reference. Various user options and easy-to-use

features make the devices suitable for various modern

DAC applications.

Application examples include:

• Decreasing Output Step Size

• Building a “Window” DAC

• Bipolar Operation

• Selectable Gain and Offset Bipolar Voltage Output

• Designing a Double-Precision DAC

• Building Programmable Current Source

• Serial Interface Communication Times

• Power Supply Considerations

• Layout Considerations

8.2.1

DC SET POINT OR CALIBRATION

A common application for the devices is a

digitally-controlled set point and/or calibration of

variable parameters, such as sensor offset or slope.

For example, the MCP48FVB2X provides 4096 output

steps. If voltage reference is 4.096V (where Gx = ‘0’),

the LSb size is 1 mV. If a smaller output step size is

desired, a lower external voltage reference is needed.

8.2.1.1

Decreasing Output Step Size

If the application is calibrating the bias voltage of a

diode or transistor, a bias voltage range of 0.8V may be

desired with about 200 µV resolution per step. Two

common methods to achieve small step size are:

• Using lower V

REF pin voltage: Using an external

voltage reference (VREF) is an option if the

external reference is available with the desired

output voltage range. However, occasionally,

when using a low-voltage reference voltage, the

noise floor causes a SNR error that is intolerable.

• Using a voltage divider on the DAC’s output:

Using

voltage

divider

provides

some

advantages when external voltage reference

needs to be very low or when the desired output

voltage is not available. In this case, a larger

value reference voltage is used while two

resistors scale the output range down to the

precise desired level.

Figure 8-2 illustrates this concept. A bypass capacitor

on the output of the voltage divider plays a critical

function in attenuating the output noise of the DAC and

the induced noise from the environment.

FIGURE 8-2:

Example Circuit Of

Set-Point or Threshold Calibration.

EQUATION 8-1:

VOUT AND VTRIP

CALCULATIONS

VCC+

VCC–

VOUT

SPI

4-wire

VREF

Optional

MCP48FVBXX

VDD

RSENSE

Comp.

VDD

VTRIP

trip

OUT

--------------------







VOUT = VREF • G •

DAC Register Value

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